1. Field of Invention
The present invention pertains to the field of ferromagnetic tunnel junctions. More particularly, this invention relates to ferromagnetic tunnel junctions having enhanced magneto-resistance.
2. Art Background
Ferromagnetic tunnel junctions may be employed in a variety of magnetic systems. For example, a ferromagnetic tunnel junction may be employed as a magnetic flux sensor. Such a magnetic flux sensor is useful in a recording head for sensing the magnetic flux emanating from a magnetic media such as a tape or disk. In addition, ferromagnetic tunnel junctions may be employed as the magnetic storage cells in a magnetic memory.
Prior ferromagnetic tunnel junctions are usually formed with a pair of ferromagnetic layers separated by a thin insulating layer. Typically, an electrical potential bias applied to the ferromagnetic layers causes the migration of electrons between the ferromagnetic layers through the insulating layer. The phenomenon that causes the migration of electrons through the insulating layer may be referred to as quantum mechanical tunneling or spin tunneling.
Typically, the resistance of a ferromagnetic tunnel junction varies according to the relative orientations of magnetic moments in the ferromagnetic layers. A tunnel junction is usually in a low resistance state when the relative orientations of magnetization are parallel. Conversely, a tunnel junction is usually in a high resistance state when the relative orientations are anti-parallel.
A useful metric for characterizing a tunnel junction is its tunneling magneto-resistance (TMR). The TMR for a tunnel junction may be defined as its resistance in the high resistance state minus its resistance in the low resistance state divided by its resistance in the low resistance state. The TMR indicates a percentage change between the high and low resistance state of a tunnel junction.
It is usually desirable to employ tunnel junctions with relatively high TMRs. For example, a magnetic memory cell having a tunnel junction with a relatively high TMR usually provides increased signal-to-noise ratio during read operations in comparison to a magnetic memory cell having a tunnel junction with a relatively low TMR. In addition, a magnetic recording head having a tunnel junction with a relatively high TMR typically yields increased sensitivity in comparison to a magnetic recording head having a tunnel junction with a low TMR.
Ferromagnetic tunnel junctions with enhanced magneto-resistance are disclosed. An enhanced magneto-resistance is realized in a tunnel junction having a topography that maximizes spin tunneling from areas of ferromagnetic crystalline grains having high polarization and minimizes the effects of defect scattering at grain boundaries. The topography of this tunnel junction includes a first ferromagnetic layer with domed areas and intervening low areas and an insulating layer having a substantially planar upper surface. The domed areas of the first ferromagnetic layer combined with the planar upper surface of the insulating layer provide a path for electron migration between the first ferromagnetic layer and a second magnetic layer that is less above the domed areas than above the low areas. This tunnel junction structure minimizes the contribution to tunneling current from grain boundaries, i.e. low areas, and increases contribution from domed areas. In addition, an enhanced magneto-resistance is realized in a tunnel junction having thin interface layers that enhance magnetic polarization properties of ferromagnetic materials near the interfaces to the insulating layer in the tunnel junction.
Other features and advantages of the present invention will be apparent from the detailed description that follows.